Scanning probe microscopes (SPMs) are used to investigate the surfaces of matter in the micrometer, nanometer, and sub-Angstrom scale. Such microscopes operate by having a probe with a sharp tip located in near contact with a surface to be profiled. In the case of the scanning force microscope the tip may actually be in contact with the surface to be profiled.
With the tip in or nearly in contact with a surface, the probe microscope uses some phenomenon to sense the surface proximity. In the case of the scanning tunneling microscope (STM) a small bias voltage is applied between the tip and the sample. The amount of resulting "tunneling" current indicates the proximity of the surface to the probe tip. This current decreases exponentially as the tip-to-sample distance increases.
In the case of the atomic force microscope (AFM), the tip senses the interatomic forces present between the tip atoms and the surface atoms to provide an indication of the surface proximity to the tip. U.S. Pat. No. RE 33,387 describes such an AFM.
Atomic force microscopes typically have the probe tip mounted on a cantilever arm. The cantilever arm has such a small spring constant that typically one nanonewton of force will cause a detectable deflection. The cantilever arm then deflects due to atomic forces present between the tip and the sample. The probe may be either attracted to or repelled by the surface, depending upon the forces at work. When relative X and Y motion exits between the tip and sample surface, distance changes in Z will result when high or low surface features pass under the tip.
The tunneling microscope may use a current-to-voltage amplifier to convert the tunneling current to a suitable voltage. This voltage may then be subtracted from a constant voltage signal, referred to as the setpoint signal. This setpoint signal establishes the separation distance between the tip and the sample surface during the scan.
Typical prior art scanning force microscopes are described in U.S. Pat. Nos. 4,724,318 and 4,800,274. The prior art also teaches that either the sample or the probe may be attached to a movable positioning device, typically a piezoelectric cylinder, which can move either the sample or the probe back and forth in the X and Y directions to create a relative rastering motion as between the probe tip and sample surface. Further, as the tip traverses over the surface in the X and Y directions, the tip may be raised and lowered in order to keep it either at a nearly constant distance from the surface or resting on the surface at a nearly constant force. This is accomplished by using the signals generated by the cantilever arm deflection and a device for detecting cantilever deflection. Control of the Z direction, or height is made possible by using a servo control device such as is shown and described in U.S. Pat. No. 4,954,704, in which a method of accomplishing this control is taught.
Unfortunately the fine motion positioning device ordinarily has a very limited vertical range, typically on the order of 25 micrometers or less. Also, in order to obtain maximum dynamic range, it is desirable that the fine motion controller be positioned at the desired distance from the sample surface when the fine motion controller is at its midrange position.
The vertical position of the probe is controlled to be the point where the detection device outputs a signal whose magnitude is equal to a setpoint signal. A servo system then creates an error signal which is the difference between the setpoint value and the detector signal. The servo system creates a control signal which is applied to the fine motion control device such that the control device maintains the probe at a constant distance from the surface. Consequently, the distance between the probe tip and the sample surface remains essentially constant even though the topography of the sample under the probe tip is changing. The correction signal then represents the surface profile.
In scanning probe microscopes a "coarse approach" system is typically used to bring the probe sufficiently close to the sample surface so that the fine motion positioner is within its vertical range. U.S. Pat. No. 4,999,495 shows coarse and fine positioning devices. The coarse approach system may include a tripod in which the legs are threaded into the housing of either the fine motion controller or the sample holder. One or more legs are then rotated such that the distance between the probe and the sample surface is reduced as described in U.S. Pat No. 5,103,095. Another coarse approach system is described in U.S. Pat No. 5,325,010. In this patent, three legs are deformed slowly and then rapidly straightened. Since the legs are on circular ramps, this effectively changes the distance between the probe and the sample. Alternatively, probe microscopes may use a single, fine pitch screw to decrease the distance between the sample and the probe such as is described in U.S. Pat. No. 4,877,957.
In the devices described above, the approach method sets the fine motion controller at its midrange and then activates the coarse approach mechanism until the detector output value reaches the setpoint value. In this prior art method, there is always the danger that the coarse approach mechanism will not stop when the detector signal reaches the setpoint value. This may be due to inertial effects or delays in the mechanical or electrical components.
In other scanning force microscopes, the fine position control mechanism may be used to modulate or vary the force applied by the probe tip to the sample surface. Such a device is described in U.S. Pat. No. 5,237,859.